FIG. 1 shows a schematic diagram illustrating the structure of a general color cathode ray tube. As shown in FIG. 1, the color cathode ray tube generally includes a glass envelope having a shape of bulb and being comprised of a faceplate panel 10, a tubular neck, and a funnel 20 connecting the panel 10 and the neck.
The panel 10 comprises faceplate portion and peripheral sidewall portion sealed to the funnel 20. A phosphor screen 30 is formed on the inner surface of the faceplate portion. The phosphor screen 30 is coated by phosphor materials of R, G, and B. A multi-apertured color selection electrode, i.e., shadow mask 40 is mounted to the screen with a predetermined space. The shadow mask 40 is hold by a peripheral frame 70. An electron gun 50 is mounted within the neck to generate and direct electron beams 60 along paths through the mask to the screen.
The shadow mask 40 and the frame 70 constitute a mask-frame assembly. The mask-frame assembly is joined to the panel 10 by means of springs 80.
The cathode ray tube further comprises an inner shield 90 for shielding the tube from external geomagnetism, a reinforcing band attached to the sidewall portion of the panel 10 to prevent the cathode ray tube from being exploded by external shock, and external deflection yokes 110 located in the vicinity of the funnel-to-neck junction.
The electron beams generated by the electron guns are deflected in both vertical and horizontal directions by the deflection yokes 110. The electron beams are selected depending on the colors by the shadow mask and impinge on the phosphor screen such that the phosphor screen emits light in different colors. Typically, about 80% of the electrons from the electron guns 50 fail to pass through the apertures of the shadow mask 40. The 80% electrons impinge upon the shadow mask 40, producing heat and raising temperature of the mask 40.
FIG. 2 shows a perspective view of a quarter of a shadow mask illustrating thermal distribution of the surface of the mask due to the impingement of electrons. As shown in FIG. 2, temperature of the mask is different for different portion of the mask. In FIG. 2, center portion of the mask has higher temperature than corner portion. The reason why the corner portion has lower temperature is that the heat at the corner portion is dissipated through the frame attached to the mask. Since the frame is attached to the mask at the skirt portion near the corner, heat at the corner is easily transferred to outside via the frame. Because the mask is thermally expanded, position of the apertures at the shadow mask is accordingly shifted from the desired position. Therefore, electron beams passing through the apertures land at the screen incorrectly. In this way the color purity at the screen is degraded. This phenomenon of purity degradation resulting from the undesired positional shift of the apertures of the mask is called the “doming effect.”
FIG. 3a shows cross sectional view of the shadow mask for illustrating purity degradation resulting from the positional shift of the apertures of the shadow mask 40. FIG. 3b is a graph showing variation of extent of positional shift of electrons landing incorrectly at the screen with respect to time after the cathode ray tube is operated.
As shown in FIG. 3a, electron beam landing at the screen is shifted due to the positional shift of the apertures of the shadow mask. As shown in FIG. 3b, the extent of the shift of the electron landing at the screen increases just after when the cathode ray tube is operated, since the temperature of the shadow mask increases. However, as heat at the shadow mask is transferred to the frame, the frame is heated and expanded. Accordingly, the positional shift of the electron landing is decreased. As the heat dissipation through the frame continues, the landing position of the electron beam is varied to the opposite direction with respect to the initial shift just after the operation of the shadow mask.
The variation of the shift of the electron beam landing causes degradation of color purity. Further, since landing position varies in accordance with the time after the shadow mask is operated, correction work of the aperture position with respect to the screen becomes difficult.
FIG. 4 is a schematic cross-sectional view of the conventional mask frame assembly. The conventional shadow mask 40 comprises a central apertured portion 401 through which electron beams pass, a non-apertured border portion 402 surrounding the apertured portion 401, and a peripheral skirt portion 403 bent back from the border portion and extending backward from the apertured portion 401.
According to the conventional mask frame assembly, the bottom portion 405 of the frame 404 intercepts electron beam 60 directed to the non-apertured border portion 402. Since electrons are blocked by the frame 404 before they impinge on the border portion, temperature elevation of the border portion is relatively small in comparison with that of the central portion of the mask. Therefore, non-uniformity of thermal expansion across the shadow mask is increased. Accordingly, the conventional shadow mask suffers from color purity degradation caused by the doming effect.
Also, improvement of the material used for the shadow mask was suggested. Invar material having low thermal expansion rate was used for the shadow mask instead of AK material. However, the result of using the invar material was not so satisfactory in view of the price of the material.